1. Field of the Invention
The present invention relates to a compatible optical pickup, and more particularly, to a compatible optical pickup which can correct a spherical aberration generated due to the difference in thickness between recording media by using a divergent light beam with respect to a recording medium having a thickness deviated from optimal design conditions of an objective lens.
2. Description of the Related Art
In general, an optical recording and/or reproducing apparatus records information on or reproduces information from an optical disc by using a light spot focused by an objective lens. In the optical recording and/or reproducing apparatus, a recording capacity of the optical disc is determined by a size S of a light spot formed by the optical recording and/or reproducing apparatus. The size S of a light spot is proportional to the wavelength λ of a light beam and is inversely proportional to the numeric aperture (NA) of the objective lens, as shown in Mathematical Formula 1 below.S∝λ/NA  Mathematical Formula 1Thus, to reduce the size S of the light spot formed on the optical disc so as to increase the density of the optical disc, it is necessary to use a short wavelength light source, such as a blue-violet laser, and to use an objective lens having an NA of 0.6 or more.
Also, when the inclination angle, refractive index, and thickness of an optical disc is θ, n, and d, respectively, and the numerical aperture of the objective lens is NA, a coma aberration W31 generated by the inclination of the optical disc can be expressed in Mathematic Formula 2 below.
                              W          31                =                              -                          d              2                                ⁢                                                                      n                  2                                ⁡                                  (                                                            n                      2                                        -                    1                                    )                                            ⁢              sin              ⁢                                                          ⁢              θ              ⁢                                                          ⁢              cos              ⁢                                                          ⁢              θ                                                      (                                                      n                    2                                    -                                                            sin                      2                                        ⁢                    θ                                                  )                                            5                /                2                                              ⁢          N          ⁢                                          ⁢                      A            3                                              Mathematical        ⁢                                  ⁢        Formula        ⁢                                  ⁢        2            Here, the refractive index n and thickness d of the optical disc indicate the refractive index and thickness of an optical medium from a light incident surface to a recording surface, respectively.
In consideration of Mathematical Formula 2, the thickness of the optical disc needs to be reduced in order to secure allowance with respect to the inclination of the optical disc while allowing the NA of the objective lens to be increased to provide higher density discs.
The thickness of the optical disc has been reduced from 1.2 mm in a compact disc (CD) to 0.6 mm in a digital versatile disc (DVD). In the future, the thickness will probably be reduced to 0.1 mm in a next generation DVD, a so-called HD-DVD (high density digital versatile disc) whose density is higher than the DVD. Of course, the NA of the objective lens has increased from 0.45 in the CD to 0.6 in the DVD. In the next generation DVD (i.e., the HD-DVD), the NA of the objective lens will probably be more than 0.6 and could be, for example, 0.85. Also, considering the recording capacity, it is highly likely the next generation DVD will use a blue-violet light source. Here, the compatibility with the existing optical discs is considered in the development of optical discs having new specifications.
For example, since the reflectivity of a DVD-R or a CD-R, which can record once, is sharply reduced according to a wavelength, it is necessary to use light sources having a wavelength of 650 nm and 780 nm. Thus, considering compatibility with the DVD-R and/or CD-R, an optical pickup for the next generation DVD needs to use two or three light sources having different wavelengths. When a compatible optical pickup uses a plurality of light sources having different wavelengths, it is preferable to use a single objective lens considering a variety of merits such as the size, easiness in assembly, and cost of an apparatus. When optical discs having different thicknesses are compatibly recorded and/or reproduced with a single objective lens, a spherical aberration is generated due to the difference in thickness between the optical discs. Thus, the compatible optical pickup should correct the spherical aberration so that optical discs having different thicknesses can be recorded and/or reproduced by using a single objective lens only.
In Korean Patent Application No.1997-42819 and corresponding Korean Patent Publication No. 1999-19439, the present applicant has suggested a CD and DVD compatible optical pickup having an optical arrangement in which an objective lens is optimally designed with respect to a DVD. Thus, a first light beam having a wavelength of 650 nm for the DVD is incident on the objective lens as a parallel beam. A second light beam having a wavelength of 780 nm for a CD and having a thickness of a substrate out of a range of design conditions of the objective lens is incident on the objective lens as a divergent beam. As such, the spherical aberration due to the difference in thickness of a substrate between a DVD and a CD can be corrected when the CD is used. However, when the divergent beam is used as in the above device, during reproduction of an eccentric optical disc, the coma aberration is greatly generated when the objective lens is shifted in a radial direction of the optical disc. Thus, a tracking error signal and a reproduction signal are further deteriorated when reproducing the eccentric optical disc.
In the configuration of a compatible optical pickup capable of compatibly using a next generation DVD and an optical disc having a density lower than the next generation DVD (such as a CD and/or a DVD), while no such device has been created or suggested, assuming that the objective lens is designed to be suitable for the next generation DVD having the highest density and a divergent beam is used for the CD and/or DVD such that the spherical aberration can be corrected and working distance can be secured, an additional problem would remain. Specifically, when a divergent beam is used in an optical pickup for a next generation DVD family optical disc (hereinafter, referred to as the next generation DVD) as a method to compatibly use the existing DVD family optical disc (hereinafter, referred to as the DVD) and the CD family optical disc (hereinafter, referred to as the CD), although the spherical aberration due to the wavelength and the difference in thickness of the optical discs could be corrected, an optical path difference (OPD) would be created due to the shift of the objective lens in a radial direction of the optical disc when an eccentric optical disc is being reproduced. In particular, the coma aberration would be generated greatly. The created OPD deteriorates a tracking error signal and a reproduction signal.
That is, a high level of technology is required to design and manufacture an objective lens capable of using a next generation DVD, a DVD, and/or a CD as a single lens. Also, it is difficult to make long working distances to be the same as that of an objective lens for a DVD that is designed optimal to the DVD.
According to technology in the field to which the present invention pertains, the objective lens is typically designed to have a working distance of about 0.6 mm with respect to a blue-violet light source and a thickness of 0.1 mm. When light spots are formed on the recording surfaces of a DVD and a CD by focusing a light beam having a 650 nm wavelength for the DVD and a light beam having a 780 nm wavelength for the CD using the objective lens having a high NA designed to the thickness of 0.1 mm and the blue-violet light beam, the working distances are 0.32 mm and −0.03 mm, respectively. That is, the CD and the objective lens collide.
However, if the light source for a CD is arranged at a position deviated from a position of the focal point of a collimating lens (i.e., a position shorter than the focal length) so that the light beam having the 780 nm wavelength is incident on the objective lens in the form of a divergent beam, the working distance can be secured and the spherical aberration due to the difference in thickness between the next generation DVD and the CD and the difference in wavelength therebetween can be corrected. Also, when a light beam for the DVD is divergent when incident on the objective lens, the spherical aberration due to the difference in thickness between the next generation DVD and the DVD and the difference in wavelength therebetween can be corrected.
However, when the divergent beam is used for the CD and/or DVD, although it is possible to correct the spherical aberration and secure a sufficient working distance, the optical pickup becomes a finite optical system. Thus, when a shift of the objective lens in a radial direction of the optical disc is generated, such as when an eccentric optical disc is being recorded or reproduced, the coma aberration is greatly generated so that the aberration characteristic is abruptly deteriorated. In particular, the deterioration of the tracking signal and the reproduction signal becomes severe due to the coma aberration.